Adding data source for buildkite_cluster

[lkerford]

We are trying to set the cluster on a pipeline which was generated through Terraform. We found a field cluster_id on the buildkite_pipeline resources (added in this PR). We were not able to find the matching data source for querying the ID of the cluster, similar to how there is a buildkite_team data source. Would it make sense for that to be added?

[jeremybumsted]

@lkerford Thank you for raising this issue. We'll take a look into this one and see what we can do 😄